1. Field of the Invention
The present invention generally relates to a device and method for limiting peaks of a signal and, more particularly, to a device and method for reducing the peak-to-average ratio (“PAR”) of a composite signal including at least two carrier signals at respective frequency bandwidths.
2. Description of Related Art
RF power amplifiers are widely used to transmit signals in communication systems. Ideally, the power amplifier would provide a uniform gain throughout a dynamic range so that the output signal of the amplifier is a correct, amplified version of an input signal. In reality, however, power amplifiers do not exhibit perfect linearity; i.e., they introduce distortion (e.g., non-linear amplitude distortion and non-linear phase distortion). Whether the power amplifier is operating in a linear or nonlinear region may depend on the amplitude of the input signal. If the input signal has an amplitude that causes the power amplifier to operate outside the linear regions, the power amplifier may introduce nonlinear components or distortion into the signal.
By reducing the peaks of an input signal to the power amplifier, the peak-to-average ratio (“PAR”) of the input signal may be reduced, thereby increasing the average power output by the power amplifier. Put differently, a power amplifier operates with increasing efficiency as the input signals to be amplified have a decreasing PAR. Therefore, techniques have been implemented to reduce the PAR of the input signal, thereby increasing the efficiency of the power amplifier.
One conventional method for reducing the PAR of an input signal is “clipping.” As the name implies, this method involves clipping peaks of the input signal to a threshold lower than a desired PAR, followed by filtering. By clipping the signal peaks, a noise-like signal is added to the input signal, thereby generating a clipped signal. When the input signal is clipped, the effect in the frequency domain is to add the spectrum of a noise-like signal to the input signal spectrum (i.e., “in-band” noise).
Although clipping techniques are generally thought to provide acceptable results, they are not without shortcomings. For example, it has been found that the signal peaks may return after filtering the clipped signal. Accordingly, it becomes necessary to again detect and clip the signal peaks to a new limit lower than the detection threshold.
Another shortcoming is that conventional clipping techniques may introduce undesirable spectral features in the signal to be transmitted; e.g., the clipped signal may deviate from its original waveform. In this respect, the clipping techniques fail to adequately address the problems of reducing the PAR of an input signal while also preserving the signal integrity within the error vector measurements (“EVM”) of the applicable technology standard (e.g. CDMA, UMTS, or other existing and future standards) for the receiver of the transmitted signal.
Another shortcoming associated with “clipping” techniques (as well as all other PAR reducing techniques) is that they keep the frequency spectrum of the noise-like signal within the frequency spectrum of the useful signal. This “in-band” noise contributes to the EVM, which (from a conceptual standpoint) may be thought of as the in-band distortion of the transmit signal.